You can't send money to Cash App through Zelle, but you can link a bank account as a workaround. Link the same bank account to your Zelle and Cash App accounts. Transfer your Cash App balance to your bank account, and then send that balance with Zelle.
If your bank or credit union offers Zelle - You can send money to people you know and trust with a bank account in the U.S. If your bank or credit union does NOT offer Zelle- The person you want to send money to, or receive money from, must have access to Zelle through their bank or credit union.